There is considerable current interest in designing modified hemoglobins for potential use as blood substitutes. Erythrocyte-free hemoglobin is an attractive candidate for a "synthetic" oxygen carrier, but the native protein suffers from two major problems: the oxygen affinity is too large (because phosphates that lower affinity in the red cell are not present in the blood stream) and the dissociation of the hemoglobin tetramer into dimers leads to rapid elimination through the renal system. Hemoglobin also fascinates theorists because of the linkage between chemical bonding on oxygenation, large structural changes within the molecule, and physiological control of oxygen transport. As in many biological systems, phenomena in both the classical and quantum domains are important. By the standards of current computational chemistry, hemoglobin is a very large system, but compared to many other allosteric proteins it is rather small. It is an ideal test case for studying the integration of these different physical levels of detail. We have been studying the conformational changes of hemoglobin on oxygenation as preparation for long time scale restrained molecular dynamics studies of the thermodynamics of hemoglobin and modified hemoglobins. We have developed a new method for identifying rigid structural elements in proteins which undergo conformational change [1], and have found that the o;-~ dimer has a large rigid core that comprises nearly half of the molecule. We also find separate rigid bodies which move relative to the core. The o: heme pocket is a rigid body, but the i3 heme pocket is not. During the past year this work has been extended to identify seven tertiary substructures within the o'-(3 dimer which describe all of the structural changes within the dimer [2]. Analysis of the coupiing between dimers is in progress. This analysis provides the necessary framework to attempt studies of the coupling between the primary events on oxygen binding and the large allosteric structural changes which occur throughout the molecule. Preliminary density functional quantum mechanical studies of porphyrin systems using deMon have failed to converge. A collaborator, Dr. Kim Baldridge of SDSC, has developed density functional capabilities in GAMESS which appear more stable; we are evaluating these. Dr. Jan Andzelm of MSI (formerly Biosym) has, as part of the NBCR, made improvements to DMol a density functional quantum mechanics code, which increase its performance and stability. This code is presently being tested on a porphyrin structure. Both the new GAMESS and the new DMoI have given markedly improved results over previous versions on large organic test systems, so there are grounds for optimism. The objectives of the quantum mechanical studies are limited to geometric optimization and vibrational properties of the molecule, which are the important factors in determining the coupling between oxygen binding at the heme and structural changes in the rest of the hemoglobin molecule. [1] Nichols W.L., Rose G.D., Ten Eyck L.F., Zimm B.H.,"Rigid domains in proteins: an algorithmic approach to their identification", Proteins 23, 38-48, 1995. [2] Nichols W.L., Zimm B.H., Ten Eyck L.F.,"Conformation-invariant structures of the (x-i ~-l human hemoglobin dimer", In Preparation, 1996.